1. Field of the Invention
The present invention relates to a cathode ray tube (CRT), and more particularly, to an enhanced CRT structure capable of shielding electromagnetic waves.
2. Description of the Related Art
Recently, the hazards of electromagnetic waves emitted from monochromatic or color CRTs on human beings have become known, thus resulting in increases in restrictions on emissions of electromagnetic waves. In order to effectively cope with such restrictions, it is essential to be cost competitive by minimizing the cost required for shielding electromagnetic waves and an antistatic effect, as well as to maximize the effect of shielding the electromagnetic waves and the antistatic effect.
According to restrictions on emission of electromagnetic waves set by the Swedish Confederation of Professional Employees (TCO), a transparent conductive layer must satisfy the following characteristics.
Thus, resistance of a transparent conductive layer should be 109 xcexa9/cm2 as an antistatic layer. Also, a transparent conductive layer as an electromagnetic wave shielding layer should have a resistance of 103 xcexa9/cm2, a hardness of 5H or more and a transparency of 95% or more.
A transparent conductive layer satisfying the above conditions is formed of a metal such as platinum (Pt), gold (Au) or indium tin oxide (ITO) in the form of a thin film.
FIG. 1A shows an example of a conventional CRT on which a transparent conductive layer for shielding electromagnetic waves coats a screen.
As shown in FIGS. 1A and 1B the conventional CRT includes a panel 13 having a screen 12 with a fluorescent layer (not shown) at the inner surface, a funnel 14 connected to the panel 13, an electron gun 15 inserted in a neck portion of the funnel 14 and a deflection yoke 16 installed around a cone portion of the funnel 14. Also, a transparent conductive layer 17 is formed of ITO with a resistance lower than 103 xcexa9/cm2 on the outer surface of the screen 12. The transparent conductive layer 17 is electrically connected to an implosion band 18 attached to a contact area between the panel 13 and the funnel 14. An external conductive layer 19 is formed on the outer surface of the funnel 14.
In the above CRT, electron beams emitted from the electron gun 15 are deflected by the deflection yoke 16 to land on the fluorescent layer. The electron beams excite a fluorescent material forming the fluorescent layer.
The electromagnetic waves from the deflection yoke 16 during the above operation are shielded by the transparent conductive layer 17 on the screen 12 and the external conductive layer on the external surface of the funnel 14, thereby suppressing emission of the electromagnetic waves outside the CRT.
For effective shielding of the electromagnetic waves, the transparent conductive layer 17 coating the screen 12 must have a resistance lower than 103 xcexa9/cm2. However, forming the transparent conductive layer 17 of a material with such a range of resistance, e.g., ITO, raises the manufacturing cost of the CRT. Thus, it is not practical to use such an expensive material for the transparent conductive layer which shields the electromagnetic waves and provides an antistatic effect.
In considering the problem, a method using ITO with a resistance higher than 103 xcexa9/cm2 or a method of applying an inverse pulse voltage to an external conductive layer synchronized with a deflection signal to cancel the electromagnetic waves, has been adopted.
However, using an ITO layer with a resistance of above 103 xcexa9/cm2 does not provide a satisfactory shielding of the electromagnetic waves and the method of applying the inverse pulse voltage to the external conductive layer requires an extra circuit for applying the inverse pulse voltage and may increase the emission of the electrons if such applied voltage is not synchronized with the deflection of the deflection yoke.
To solve the above problems, it is an object of the present invention to provide a cathode ray tube (CRT) capable of improving the shielding effect on the electromagnetic waves and the antistatic effect, as well as lowering the cost which may be raised by forming a transparent conductive layer.
To achieve the above object, there is provided a cathode ray tube (CRT) comprising: a panel with a screen; a funnel connected to the panel, having a cone portion and a neck portion; an electron gun inserted in the neck portion; a deflection yoke installed around the cone portion; an external conductive layer deposited on the external surface of the funnel; a transparent conductive layer deposited on the outer surface of the screen, having a resistance greater than 1xc3x97105 xcexa9/cm2 and equal to or less than 9xc3x97105 xcexa9/cm2; and a conductive ground portion electrically connected to the external conductive layer and attached to the cone portion of the funnel, facing the deflection yoke, being extended toward the neck portion.
Preferably, the transparent conductive layer is formed of indium tin oxide (ITO).